Radio frequency identification (RFID) systems are widely used for identifying and tracking items, inventory control, supply chain management, anti-theft of merchandize in stores and many other applications. A typical RFID system consists of a plurality of RFID transponders to be interrogated by RFID transceivers or RFID readers. Typically, an RFID reader interrogates one or more of the transponders over a wireless forward link, such as an UHF signal. The transponders respond to the readers' interrogations by transmitting tag information back to the reader over a wireless return link.
There exist active and passive transponders depending on how they are powered. A passive transponder does not have its own power source. Electrical power must be derived from the RF interrogating signal. For this a passive RFID transponder typically includes a loop antenna tuned to receive an RF interrogating signal. The loop antenna is electrically connected to a rectifier of the RFID transponder. The RF interrogating signal induces an AC signal within the loop antenna that is provided to the rectifier. The rectifier rectifies and amplifies the voltage of the AC signal to charge a storage capacitor and/or to power digital circuitry of the transponder.
Common architectures of RFID transponders include a Dickson-style charge pump and a multi-stage rectifier comprising a cascade of numerous rectifier stages. A schematic illustration of a rectifier stage implemented with MOS devices is for instance shown in FIG. 1. A rectifier stage according to the prior art comprises an input node Vin, an input capacitor Ci and two metal-oxide-semiconductor field-effect transistors (MOSFETs). Here, the drain of an NMOS transistor N1 is connected to the drain of a PMOS transistor P1. The gate of the NMOS transistor N1 is connected to the source of said transistor. Likewise, also the gate of the PMOS transistor P1 is connected to the source of said PMOS transistor. The source of the PMOS transistor P1 is connected to the output node Vdc of the rectifier stage, which is also connected to ground via an output capacitor Co.
Use of MOSFETs instead of conventional diodes generally improves efficiency of such rectifiers and enables a good integration of the rectifier into CMOS processes.
In order to provide a highly efficient rectifier, a forward resistance of the rectifier stage must be of rather low impedance to reduce resistance and voltage losses while the input signal polarity is correct in order to charge up a main charge pump. When the signal polarity is not correct to charge the output of the rectifier, the rectifier stage should provide a high impedance in the reverse direction to prevent discharging of the charge pump. With the implementation of MOSFETs, this is a balancing act since there is not an abrupt on/off point of the MOSFETs.
With the implementation of a MOSFET-based rectifier stage, a certain voltage level at the input is always required to turn on the MOSFETs. A signal swing at the input must be sufficiently large to turn on NMOS and PMOS transistors. In particular, a peak-to-peak AC swing of the input signal derived from the antenna must exceed the intrinsic threshold voltage of the MOS devices before a rectification can occur. This is a known drawback for low power operated transponders that are hence of high sensitivity.
The U.S. Pat. No. 7,944,279 B1 describes a charge pump stage having voltage biased diodes and utilizing a VT-cancellation technique. The charge pump stage includes rectifier diodes which are voltage biased using an auxiliary charge pump and a voltage clipper diode. Because of a low power operation the voltage clipper has relatively higher impedance. The charge pump stage functions less efficiently at higher input power levels because of the higher impedance of the voltage clipper resulting from a conduction angle of rectifier diodes exceeding Tr. In another embodiment, said document describes a tunable charge pump stage of an RFID tag. The charge pump stage includes an RF node, a capacitor bank, current-biased rectifier stages, a programmable current source, a control circuit, a test pad and a DC bus.
There, the control circuit controls the capacitor bank and programs the programmable current source to provide various biases to the rectifier stages. Such an implementation is rather complicated and expensive.
It is hence commonly known to make use of secondary or auxiliary charge pumps to lower the turn on voltage of MOS devices of a rectifier stage, thus lowering a peak-to-peak AC swing required at the rectifier input to provide rectification.
So for each MOSFET of a rectifier stage, an auxiliary charge pump can be used to generate a rather fixed DC voltage near the turn on voltage of the respective MOSFET. This biases the respective MOSFET at the edge of conductivity so that a large signal on the input is not required to conduct rectification. In general this solution works fine as long as the input voltage level is substantially constant.
For reading and writing a memory of a RFID transponder significantly different input voltage levels have to be provided. As the input voltage increases, for example to conduct a writing procedure, the operating point of the rectifier stage will move. An increasing input voltage may lead to a larger bias of the MOS devices of the rectifier stage thereby increasing the leakage current when the rectifier is supposed to be off. Once there is a significant power provided by the antenna, a decrease in efficiency is tolerable since the signal is large enough. In particular for low input power, it is desirable to increase the efficiency of the rectifier for both, a read input power level and a write input power level.